U.S. patent number 6,274,028 [Application Number 09/298,284] was granted by the patent office on 2001-08-14 for electrolytic wastewater treatment method and apparatus.
Invention is credited to Clyde Kuen-Hua Hu, Patrick Pei-Chih Hu, Paul Pei-Yung Hu.
United States Patent |
6,274,028 |
Hu , et al. |
August 14, 2001 |
Electrolytic wastewater treatment method and apparatus
Abstract
A method and apparatus for purifying aqueous effluent streams to
reduce chemical oxygen demand thereof, where the method comprises
direct oxidation of water-soluble organic material in an
electrochemical cell that incorporates stainless steel electrodes,
whose stability and lifetime are enhanced by inclusion of
circulating metal chips.
Inventors: |
Hu; Clyde Kuen-Hua (Taipei,
TW), Hu; Paul Pei-Yung (Hong Kong, HK), Hu;
Patrick Pei-Chih (Durham, NC) |
Family
ID: |
23149846 |
Appl.
No.: |
09/298,284 |
Filed: |
April 23, 1999 |
Current U.S.
Class: |
205/754;
205/753 |
Current CPC
Class: |
C02F
1/46114 (20130101); C02F 1/4672 (20130101) |
Current International
Class: |
C02F
1/467 (20060101); C02F 1/461 (20060101); C02F
001/46 () |
Field of
Search: |
;205/742,746,747,751,753,754,761 ;204/252,263,264,275.1,276 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gorgos; Kathryn
Assistant Examiner: Feely; Michael J
Attorney, Agent or Firm: Hultquist; Steven J.
Claims
What is claimed is:
1. An electrolytic oxidation process for purifying wastewater by
oxidation of organic and oxidizable inorganic substances contained
therein, said process comprising:
flowing the wastewater into an electrolytic oxidation cell, where
the cell comprises a stainless steel anode and cathode and contains
iron chips, said chips being in electrical contact with the anode
and prevented from making electrical contact with the cathode by a
non-electrically-conductive, liquid-permeable barrier;
applying a voltage across the electrodes to energize the
electrolytic oxidation cell and effect electrolytic oxidation of
organic and oxidizable inorganic substances in the wastewater;
and
discharging from the electrolytic oxidation cell a treated
wastewater having a reduced COD content.
2. The process of claim 1, wherein the voltage applied across the
electrodes of the electrolytic cell produce a current of from about
2 to about 20 amperes in the electrolytic cell.
3. The process of claim 1, wherein conductivity of the wastewater
is adjusted to about 200 to about 2000 micro Siemens per centimeter
by adding a strong electrolyte to such wastewater before flowing
such wastewater into the electrolytic oxidation cell.
4. The process of claim 3, wherein the strong electrolyte added
into the wastewater is NaCl.
5. The process of claim 1, wherein the voltage applied across the
electrodes of the electrolytic cell produces a current of from
about 9 to 12 amperes in the electrolytic oxidation cell.
6. The process of claim 1, wherein the wastewater is characterized
by COD of from about 200 to about 2000 ppm.
7. The process of claim 1, wherein the electrolytic oxidation cell
is filled to between 80% and 95% of its volumetric capacity with
iron chips.
8. The process of claim 1, wherein the non-electrically-conductive,
liquid-permeable barrier comprises a plastic netting.
9. The process of claim 1, wherein the wastewater stream is
recirculated through the electrolytic oxidation cell.
10. The process of claim 1, further comprising flowing the
wastewater through one or more additional electrolytic oxidation
cells in sequence to discharge from a final one of said additional
electrolytic oxidation cells a further reduced COD content treated
wasterwater.
11. The process of claim 1, wherein the wastewater is derived from
an upstream process facility selected from the group consisting of
power generation stations, printed circuit board manufacturing
facilities, and landfill seepage wastewater.
12. The process of claim 1, conducted in a mode selected from the
group consisting of continuous, semi-continuous and batch modes of
operation.
13. The process of claim 1, wherein the wastewater prior to being
flowed into the electrolytic oxidation cell is subjected to pH
adjustment, to obtain a pH level of from about 7 to about 10 in the
wastewater flowed into the electrolytic oxidation cell.
14. A method of oxidation of organic and oxidizable inorganic
substances in wastewater, comprising:
flowing the wastewater into an electrolytic oxidation cell, wherein
the cell comprises an anode and a cathode and contains iron chips,
said chips being in electric contact with the anode and prevented
from making electrical contact with the cathode by a
non-electrically-conductive, liquid-permeable barrier;
applying a voltage across the electrodes to energize the
electrolytic oxidation cell and effect oxidation of organic and
oxidizable inorganic substance in the wastewater; and
discharging from the electrolytic oxidation cell a treated waste
water.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to a method and apparatus for
purifying aqueous effluent streams to reduce contamination as
measured by chemical oxygen demand, where the method comprises
direct oxidation of water-soluble organic and oxidizable inorganic
substances in an electrolytic oxidation cell that incorporates
stainless steel electrodes, and wherein the stability and lifetime
of the anode are enhanced by incorporation of metal chips.
2. Description of the Related Art
Industrial wastewater streams may be contaminated by various
substances that render their discharge into waterways or municipal
waste treatment systems problematic or illegal. Contaminants may be
organic or inorganic in nature and are often found in complex
combinations.
One widely regulated parameter is "chemical oxygen demand" (COD), a
measure of the quality of wastewater effluent streams prior to
discharge. The COD test predicts the oxygen requirement for
complete oxidation of oxidizable contaminants present in the
effluent; it is used for the monitoring and control of discharges,
and for assessing treatment plant performance. Chemical oxygen
demand is defined as the amount of oxygen in milligrams per liter
(parts-per-million, ppm) required to oxidize both organic and
oxidizable inorganic compounds that are present in the
effluent.
The United States Environmental Protection Agency (USEPA) provides
a set of standard methods to determine COD in aqueous
effluents:
Test Method USEPA Document Source Chemical Oxygen Demand - 0410.4
600/4-79-020 Colorimetric Chemical Oxygen Demand - 0410.4
600/R-93-100 Semi-Automated Colorimetric Chemical Oxygen Demand -
0410.3 600/4-79-020 Titrimetric, High Level Chemical Oxygen Demand
- 0410.2 600/4-79-020 Titrimetric, Low Level Chemical Oxygen Demand
- 0410.1 600/4-79-020 Titrimetric, Mid Level
Acceptable wastewater treatment methods must be cost-effective, and
hence a desirable method will be characterized by rapidity of
contaminant removal, stability of the process over time, low cost
of energy and consumables, and simplicity of equipment design. In
this view, electrolytic oxidation is a favorable method for
reducing the amount of organic compounds and other oxidizable
species in an aqueous effluent to a level that is acceptable for
discharge to a treatment facility. Electrolytic oxidation has
several advantages over chemical or thermal treatment techniques,
including ease of operation, simplicity of design, and relatively
small equipment space requirements. Electrolysis is also considered
to be relatively safe to operate when compared to oxidative
treatment techniques which require handling of powerful chemical
oxidants.
The electrolytic treatment of wastewater has been the subject of
much research and many patents, e.g., U.S. Pat. No. 4,445,990,
"Electrolytic Reactor for Cleaning Wastewater," issued May 1, 1984;
U.S. Pat. No. 5,516,972, "Mediated Electrochemical Oxidation of
Organic Wastes Without Electrode Separators," issued May 14, 1996;
U.S. Pat. No. 5,688,387, "Turbo Electrochemical System," issued
Nov. 18, 1997.
However there remain a number of problems associated with known
methods of electrolytic oxidation of solutes in wastewaters. An
important focus of difficulty is the lack of stable, inexpensive
anode materials.
In wastewater purification, a high oxygen overvoltage is required
at the anode for water-oxidation intermediates to be formed from
degradation of oxidation-resistant organic substances. Most anode
materials gradually corrode during use in electrolytic oxidation,
especially in harsh chemicals. Corrosion of typical anodes such as
platinum, ruthenium oxide, lead dioxide and tin dioxide results in
a lack of process stability, is uneconomical, and leads to
discharge of unacceptable toxic species into the environment.
Platinum anodes are the most acceptable of traditional electrodes,
yet in practice the rate of platinum loss from the electrode is
high enough that a metal recovery system would be required, adding
significantly to the cost and complexity of such an electrolytic
oxidation apparatus and method. Lead dioxide and graphite
electrodes are not sufficiently stable: modification by tin oxide
doping has been proposed to increase electrode lifespan, but leads
to the aforementioned problem of release of a toxic species.
Furthermore, many anode materials tend to become fouled during
electrolytic oxidation of various solutes by the formation of an
adsorbed layer of residue on the working surface of the anode. This
lowers the effectiveness and useful lifetime of the anode,
resulting in longer treatment times and more frequent
equipment-related shutdowns. An anode that is not subject to a
decrease in efficiency due to change in polarization at the
electrode surface is needed in the art.
An additional problem with conventional anode materials is lack of
energy efficiency when used in electrolytic oxidations. As a result
of such deficiencies, the wastewater treatment system requires a
relatively long time and high energy expenditure to achieve desired
results, at the electrical current densities that are typically
employed.
The development of suitable electrode materials for wastewater
treatment has long been an active area of research. Some
representative approaches are described in the following patents.
U.S. Pat. No. 4,360,417, "Dimensionally Stable High Surface Area
Anode Comprising Graphitic Carbon Fibers," issued Nov. 23, 1982,
describes anodes comprising carbonaceous fibrous materials with a
surface coating of a mixture of titanium dioxide and ruthenium
dioxide. U.S. Pat. No. 4,415,411, "Anode Coated with .beta.-Lead
Dioxide and Method of Producing Same," issued Nov. 15, 1983,
describes an anode which comprises various layers of titanium, a
platinum-group metal, and a lead dioxide coating. U.S. Pat. No.
5,399,247, "Method of Electrolysis Employing a Doped Diamond Anode
to Oxidize Solutes in Wastewater," issued Mar. 21, 1995, describes
an anode comprising electrically conductive crystalline doped
diamond. Such electrodes do not overcome the problems of high cost,
contribution of toxic species to the waste stream, and lack of
process stability due to corrosion or formation of adsorbed layers
on the electrode surface.
Electrodes that comprise particulate materials are known.
Electrodes comprising electroconductive particulates have been
described for cathodic processes such as electroprecipitation or
electrowinning, that is, the recovery of a metal by deposition of
the metal from an aqueous solution, such as a
metal-ion-contaminated wastewater or aqueous leach liquors obtained
by leaching ore. The metal to be recovered is deposited onto the
cathode to a desired thickness, and the cathode is then removed and
the metal recovered. Particulate cathodes are described, e.g., in
U.S. Pat. No. 4,692,229, "Electrode Chamber Unit for an
Electro-Chemical Cell Having a Porous Percolation Electrode,"
issued Sep. 8, 1987; U.S. Pat. No. 3,974,049, "Electrochemical
Process, issued Aug. 10, 1976; and references cited therein.
Because the process constraints of the cathodic applications for
which these electrodes are designed are quite different, such
particulate cathode materials, e.g., graphite, copper, do not have
the ability to be used as the anode in an electrolytic oxidation
and cannot be operated with high energy-efficiency and in the
presence of oxygen over-voltages, as would be required for an
oxidative wastewater purification process.
An organic or organometallic synthesis process using an anode
comprising metal particulates which are consumed in the synthesis
reaction has been described in U.S. Pat. No. 4,828,667,
"Electrolytic Cells with Continuously Renewable Sacrificial
Electrodes," issued May 9, 1989. This patent describes the
electrocarboxylation of 2-acetonaphthone with the accompanying
consumption of the anode. The electrocarboxylation process
disclosed in this reference utilizes small aluminum cylinders which
are continuously consumed and replenished by a feed device, and
involves the following electrochemical reactions:
cathode:
anode:
Anodes designed for such processes are not readily adaptable to use
in an electrolytic oxidative wastewater purification process.
Further, the aluminum anode in such system would contribute toxic
aluminum to the waste stream and would quickly become passivated by
an oxide coating.
There is thus a compelling need for a method and apparatus for
electrolytic oxidation of solutes in liquid solutions, which will
avoid or minimize the problems described above. Such a method and
apparatus will desirably have the following features: an anode
formed of a relatively inexpensive material and of relatively
simple design; an anode whose corrosion does not result in
discharge of toxic species; an anode that does not become
significantly inefficient through fouling caused by the formation
of an adsorbed layer; an anode that operates with high
energy-efficiency; and an anode whose ongoing corrosion does not
destabilize the process variables over time.
SUMMARY OF THE INVENTION
The present invention in one aspect relates to an electrolytic
purification method and apparatus for treatment of wastewaters to
reduce chemical oxygen demand, by oxidation of water-soluble
organic and other oxidizable materials contained therein. The
electrolytic purification system of the invention utilizes one or
more electrochemical cells. The cells employ stainless steel
electrodes and contain iron chips, which are mobile and circulate
freely as liquid flows through the cell. The iron chips are in
electrical contact with the anode and are prevented from making
contact with the cathode by a non-conductive but liquid-permeable
barrier. The iron chips thus provide a dynamic and fluid electrode
surface that is efficient and resistant to performance
degradation.
In the practice of the invention, a voltage, e.g., of 1-20 volts
(V), is applied across the electrodes to generate a desired
current, e.g., of 2-15 amperes (A). Electrolysis in such a cell
reduces COD in typical wastewaters by oxidizing to CO.sub.2
water-soluble organic and other oxidizable contaminants.
The invention relates in another aspect to an electrolytic
oxidation apparatus, comprising two or more electrochemical cells
of the above-described type, arranged in series for sequential flow
of wastewater therethrough to effect the desired level of COD
removal.
In one specific embodiment, the invention relates to an
electrolytic oxidation process for purifying a wastewater stream by
oxidation of water-soluble organic and oxidizable inorganic
substances contained therein, such process including the steps
of:
flowing the wastewater stream into an electrolytic oxidation cell,
wherein the cell comprises stainless steel anode and cathode and
contains iron chips, with the chips being in electrical contact
with the anode and prevented from making electrical contact with
the cathode by a non-electrically-conductive, liquid-permeable
barrier;
applying a voltage across the electrodes sufficient to produce a
current of from about 2 to about 20 A.
In one embodiment of the inventive process, the wastewater stream
is characterized by a conductivity of from about 200 to about 2000
micro Siemens per centimeter (.mu.S/cm) and COD of from about 200
to about 2000 parts per million by volume (ppm). The electrolytic
oxidation cell is preferably filled to between 80% and 95% of its
volumetric capacity with the iron chips, and the
non-electrically-conductive, liquid-permeable barrier preferably
comprises a plastic netting. The wastewater stream may be
recirculated through the electrolytic oxidation cell to achieve
desired levels of purity.
The inventive process in another aspect may comprise:
flowing the wastewater through one or more additional electrolytic
oxidation cells, correspondingly constructed to comprise stainless
steel anode and cathode elements and to contain iron chips, in
which the chips being in electrical contact with the anode and
prevented from making electrical contact with the cathode by a
non-electrically-conductive, liquid-permeable barrier;
applying a voltage across the across the electrodes of the
additional electrolytic oxidation cells sufficient to produce a
current of from about 2 to about 20 A.
The invention in another specific aspect further comprises an
electrolytic oxidation apparatus for purifying a wastewater stream
by oxidation of water-soluble organic and oxidizable inorganic
substances contained therein. Such apparatus comprises:
an electrolytic oxidation cell, where the cell comprises stainless
steel anode and cathode and contains iron chips, said chips being
in electrical contact with the anode and prevented from making
electrical contact with the cathode by a
non-electrically-conductive, liquid-permeable barrier;
means, such as a current source, power supply, generator, turbine,
power cable or other electrical power elements, for applying a
voltage across the stainless steel anode and cathode sufficient to
produce electrolytic oxidation conditions for oxidation of organic
and oxidizable inorganic substances in the wastewater;
means, e.g., including flow circuitry elements such as piping,
conduits, flow channels, connecting fittings, etc., and motive flow
devices such as pumps, compressors, impellers, ejectors, eductors,
etc., for flowing wastewater into and out of the electrolytic
oxidation cell.
Preferably the non-electrically-conductive, liquid-permeable
barrier comprises a plastic netting, but other permeable barrier
structures may be employed, such as mesh, screen, membrane or other
structures of a liquid permeable and non-conductive character, as
hereinafter more fully described.
The iron chips are preferably generally disk-shaped, but may be of
any suitable shape and size characteristics.
An electrolytic oxidation apparatus according to the invention may
further comprise one or more additional electrolytic oxidation
cells similar to the first, with means such as pump and conduit
elements to flow the wastewater from the first electrolytic
oxidation cell to the one or more additional electrolytic oxidation
cells for sequential passage through the electrolytic cells in the
apparatus system.
Various other aspects, features and illustrative embodiments of the
invention will be more fully apparent from the ensuing disclosure
and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic top view of an electrolytic oxidation cell
according to the invention.
FIG. 2 is a schematic representation of an electrolytic oxidation
wastewater treatment apparatus according to one embodiment of the
invention.
FIG. 3 is a schematic representation of an electrolytic oxidation
wastewater treatment apparatus according to another embodiment of
the invention, employing two electrolytic oxidation cells in
series.
DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS
THEREOF
The present invention relates to an electrolytic purification
apparatus and method for treatment of wastewaters to reduce
chemical oxygen demand, by oxidation of water-soluble organic and
other oxidizable materials in one or more electrochemical cells.
The desired electro-oxidation is conducted in one or more
electrolytic oxidation cells that employ stainless steel electrodes
and contain metal chips. The metal chips are mobile and circulate
freely as liquid flows through the cell, so that the metal chips
form many, ever-changing electrical contacts with the anode, but
are prevented from making electrical contact with the cathode by a
non-electrically-conductive but liquid-permeable barrier.
The metal chips thus provide a dynamic and fluid electrode surface
that is efficient and resistant to performance degradation. The
wastewater may be recirculated through the electrolytic oxidation
cell(s) for additional purification. The apparatus may optionally
include monitoring devices such as oxygen, pH, and/or conductivity
meters, or means to sample the wastewater stream for parameters
such as COD.
The electrolytic oxidation cells can be of any suitable shape and
volume, as may be readily determined based on the specific
wastewater-generating process and/or wastewater stream
characteristics involved in a particular end use application of the
invention. The cell is fabricated of any suitable material, as is
readily determinable by those of ordinary skill in the art without
undue experimentation, preferably a material that is strongly
resistant to degradation and rupture under the conditions of
use.
In one illustrative embodiment, the cell is a non-conductive
tubular container with an inside diameter of from about 1 to about
3 inches and a length of from about 1 and about 3 feet. At the top
of the tube, a flange is secured to the tubular container. The
flange has three openings. Through two openings pass stainless
steel rods which serve as the electrodes, with diameters of about 1
mm each and a distance of about 10 mm therebetween throughout their
length. The third opening in the upper flange is an outlet for the
treated liquid. At the bottom of the tube is an opening through
which untreated solution is flowed into the cell.
The stainless steel electrodes are fabricated with any suitable
dimensions proportionate to the size of the electrolytic cell
compartment. The electrodes are formed of stainless steel. The
stainless steel may be of any suitable type, e.g., 316 stainless
steel alloy or any other advantageous stainless steel composition,
as will be readily determinable by those of ordinary skill in the
art.
The metal chips are formed of iron. The "chips" may be of any shape
that is conducive to free circulation and mixing in a flowing
liquid stream and is not prone to clumping or adhesion to the
electrolytic cell wall or electrode surface. In one embodiment the
metal chips are generally disk-shaped. Suitable dimensions for such
metal chips may be readily determined without undue experimentation
by those of ordinary skill in the art. One highly preferred size of
such metal chips is about 3 mm in diameter and about 1 mm in
thickness. Other shapes of chips that may be usefully employed in
the broad practice of the invention include flakes, rings, pellet
shapes, spheres, cylindrical shapes, etc. Preferably, the chips are
flattened or planar to enhance electrical contacting in the slurry
of chips in the wastewater undergoing treatment. The chips may be
of a single dimensional size, or the chips may constitute a
population of differing sized members, as may be desired or
appropriate in a given end use application of the invention.
The cell compartment is filled with metal chips to a level that
allows the chips to mix and circulate freely in a flowing liquid
stream, but that is sufficiently concentrated in chips so that they
will be in frequent physical and electrical contact with one
another and with the anode. Desirably, the chips are able to mix
and circulate freely and form many constantly changing current
pathways, to provide a dynamic and fluid electrode surface. The
cell is functional when filled in the volumetric range of from
about 20% up to in the vicinity of 100% with metal chips, based on
the volume of the cell chamber. The preferred filling range is from
about 80% to about 95% by volume, based on the total volume of the
cell chamber.
A non-electrically-conductive but liquid-permeable barrier prevents
the metal chips from making electrical contact with the cathode.
This barrier must be sufficiently liquid-permeable that the
wastewater being treated can flow freely through the cell and
between the anode and cathode vicinities. The barrier will
preferably have openings or pores which may be regular or irregular
in shape, and will have mean diameters of a size that will allow
maximal liquid flow while preventing the through-passage of a
deleterious, i.e. short-circuit-causing, amount of the metal chips
during the operation of the electrolytic cell. The preferred
dimensions of the openings or pores will be determined based on the
size of the metal chips being used.
The non-conductive but liquid-permeable barrier most preferably is
formed of a material which is resilient to the constant impacts of
the metal chips when the cell is in operation, which is chemically
inert under the conditions of electrolytic oxidation, and which is
not electrically conductive. Examples of suitable barrier materials
include plastic netting, polymeric films (e.g., of polyvinylidene
chloride, polysulfone, polyvinylchloride, etc.), ceramic screens,
sintered glass fiber sheeting, etc.
The wastewater to be purified in the practice of the invention may
require pretreatment to remove suspended solids, to adjust pH,
and/or to adjust conductivity, prior to its introduction into the
electrolytic oxidation cell. Accordingly, the process system may
comprise an upstream clarifier or sedimentation basin, screening
unit, filter, chemical dosing chamber, biological oxygen demand
(BOD) removal treatment unit, radiation treatment chamber,
ozonation unit, or any other pretreatment unit that will
advantageously assist the processing of the wastewater in a manner
that will, in combination with the COD treatment system of the
invention, produce a final effluent of the desired discharge
quality.
For example, the upstream optional pretreatment of the wastewater
may include processing to lower the levels of suspended solids in
the wastewater, including sedimentation and/or filtration, with or
without flocculation of the wastewater.
When not in use, the electrolytic oxidation cell is stored charged
with a solution with a conductivity of from about 500 to 2000
.mu.S/cm. The solution can be a simple salt solution, e.g.
NaCl.
Prior to electrolytic oxidation treatment, the conductivity of the
wastewater is adjusted to a suitable level, e.g., in a range of
from about 200 to about 2000 .mu.cm. A preferred conductivity for
treatable wastewaters is in the vicinity of about 1500 .mu.S/cm.
The conductivity can be adjusted with any strong electrolyte, e.g.,
alkali and alkaline earth chlorides, bromides, iodides, nitrates,
perchlorates, chlorates, bromates, and alkali metal sulfates. For
reasons of cost and simplicity, NaCl is typically used to adjust
conductivity to a level sufficient to support electrolysis.
Also, prior to electrolytic oxidation treatment, the pH of the
wastewaters is suitably adjusted to a pH level of from about 7 to
about 10, preferably from about 8 to about 10. Any strong inorganic
base can be used, but for reasons of cost and simplicity, NaOH or
Na.sub.2 CO.sub.3 are typically employed.
In electrolytic oxidation of wastewater to reduce COD, fairly
complex organic molecules are frequently oxidized all the way to
CO.sub.2. In such electrodestruction reactions, many bonds are
broken and large molecular rearrangements occur. Such reactions are
often much slower than simple single-electron-transfer reactions.
The flow rate, temperature, current density and electrode potential
will all affect the rate at which complete electrolytic oxidation
of the oxidizable contents of a wastewater stream can occur.
In one illustrative embodiment of the invention using an
electrolytic cell having dimensions described hereinabove, a
voltage of 1-20 V is applied across the electrodes to generate a
current of 2-15 A. Preferably, the applied voltage is about 10 V
yielding a current of about 10 A. The wastewater stream is pumped
into the cell at a volumetric flow rate of from about 0.5 to about
5.0 liters per minute (L/min), preferably from about 0.7 to about
3.0 L/min. The wastewater may be recirculated through the cell for
a given number of times, or there may be a system to continually
feed and draw off wastewater, calculated to allow a suitable
average residence time in the cell to yield the desired reduction
in COD.
The wastewater treatment system of the invention may utilize any
suitable flow circuitry means, including pumps, fans, impellers,
etc., arranged in the flow circuit including piping, conduits,
fittings, sensors, monitors, controllers, etc., as necessary or
desirable in a given end use application of the invention to
achieve a desired level of COD reduction in a specific wastewater
being treated. The wastewater may derive from any suitable source,
such as for example an industrial manufacturing or processing
facility, mining operation, riparian streams, power generation
plants, etc.
The process of the invention may be carried out at any suitable
temperature level, and preferably is at or near ambient
temperature, e.g., temperature in the range of from about 5 to
about 40.degree. C.
The residence time of the wastewater in the electrolytic cell may
be widely varied, depending on the type of wastewater involved and
its COD content (amount of COD, and types of COD-constituting
components), as well as the equipment constraints of the system,
the degree of COD removal required, the dimensional characteristics
of the electrolytic cell(s) in the system, the volumetric flow rate
of the wastewater into the treatment system, and the current
density and other process conditions utilized in the treatment
system.
The specific conformation of equipment and the operating conditions
to be employed may be readily empirically determined, by varying
the process system variables of interest and determining the
resulting COD removal efficiency, to select the structural form and
the processing parameters of the electrolytic treatment system in a
specific embodiment.
The electrolytic treatment method of the invention may be carried
out in a continuous, semi-continuous, or batch mode, depending on
the specifics of the wastewater being treated. It may be desirable
to utilize the system of the invention in combination with holding
tanks, surge reservoirs or other storage facilities when the
wastewater flow is intermittent or highly variable in
character.
The electrolytic oxidation treatment method of the invention may
further comprise passing the wastewater through additional
electrolytic oxidation cells in a series arrangement, to obtain
more rapid treatment or higher final purities. The cells in series
may be operated from one or more current sources and may have one
or more pumping means, as appropriate for the size and flowrates
involved.
The apparatus and method of the invention are useful for treating
wastewaters with COD in the range of about 200 ppm to about 2000
ppm. After treatment according to the invention, the wastewaters
typically show COD readings that are reduced by more than 50%,
preferably by more than 80%, and even more preferably by more than
90%.
In the course of electrolytic oxidation, metal ions released from
the electrodes can be chelated by organic substances present in the
wastewaters, resulting in flocculation. The purification system can
include a means for separating the flocculate from the liquid
phase.
The floc separation may be effected in a sedimentation tank or
gravity clarifier, centrifuge, filter or other suitable separation
means, optionally with addition of a coagulant or agglomerating
agent to the floc-containing wastewater, to effect consolidation
and enhanced separation of the floc particles from the
wastewater.
Referring now to the drawings, FIG. 1 is a schematic top view of an
electrolysis cell 10 according to one embodiment of the invention.
The cell includes a chamber containing anode 1 and cathode 2
immersed in electrolyte 6. The cell is filled to about 80%-95% by
volume of its capacity with metal chips 4, which are prevented from
contacting cathode 2 by a porous barrier 3 which is itself not
electrically conductive. The anode and cathode are supplied with
current from current source 11 at a predetermined voltage.
FIG. 2 is a schematic side view of a purification apparatus
employing the electrolysis cell of FIG. 1. As in FIG. 1, the cell
comprises a chamber containing anode 1 and cathode 2 immersed in
electrolyte 6. The cell is filled to about 80%-95% of its capacity
with metal chips 4, which are prevented from contacting cathode 2
by a porous barrier 3 which is itself not electrically conductive.
The anode and cathode are supplied with current from current source
11 at a predetermined voltage. Wastewater is pumped by pump 7 from
wastewater collection tank 8 via conduits 12 and 13 to the
electrolytic oxidation cell compartment 5.
As the wastewater flows through the cell compartment, the metal
chips 4 are constantly agitated and present a dynamic surface to
the liquid phase. For purposes of maintaining such agitation, the
cell chamber may contain an impeller or a gas sparger to enhance
the degree of circulation of the water/chips slurry in the cell
chamber. Alternatively, the circulation rate of liquid through the
cell may be inherently sufficient to maintain the desired degree of
agitation or circulation of the chips in the water/chips
slurry.
When current is supplied to the anode 1 and cathode 2 from current
source 11, oxidation reactions occur at the stainless steel anode 1
and at the surfaces of the metal chips 4, which are in electrical
contact with the anode. Organic and oxidizable inorganic substances
in the wastewater are oxidized, and the treated wastewater flows to
three-way valve 9, from which depending on the valve position, it
may flow via conduit 14 to a storage tank 16 and then via conduit
17 to discharge to a waterway or a treatment facility (not shown).
Alternatively, the treated wastewater may be returned via conduit
15, collection tank 8, and conduits 12 and 13 to thereby undergo
additional oxidation cycles.
FIG. 3 is a schematic representation of an electrolytic oxidation
wastewater treatment apparatus 30 according to another embodiment
of the invention, employing two electrolytic oxidation cells in
series. Wastewater from collection tank 31 is pumped by pumps 32
and 33 through the apparatus. Wastewater flows through conduits 34
and 35 to first electrolytic oxidation cell compartment 36, where
anode 37 and cathode 38 are immersed in electrolyte 39. Anode 37 is
in electrical contact with metal chips 40, which fill the cell
compartment 36 to about 80% to 95% of its volumetric capacity, and
which are prevented from contacting cathode 38 by a
liquid-permeable barrier 41 which is not itself electrically
conductive. Current source 42 provides current to the anode 37 and
cathode 38, whereby organic and oxidizable inorganic substances in
the wastewater are oxidized.
Upon exiting the first cell, the wastewater flows via conduit 43 to
second electrolytic oxidation cell compartment 44, where the
process is repeated. Second anode 45 and second cathode 46 are
immersed in second electrolyte 47, which may be the same as first
electrolyte 39 or may be different. Anode 45 is in electrical
contact with metal chips 48, which fill the cell compartment 44 to
about 80% to 95% of its volumetric capacity, and which are
prevented from contacting cathode 45 by a liquid-permeable barrier
49 which is not itself electrically conductive. Current source 42
provides current to the anode 45 and cathode 46, whereby organic
and oxidizable inorganic substances in the wastewater are oxidized.
Alternatively, anode 45 and cathode 46 may be connected to a second
current source.
The wastewater then flows via conduit 59 to three way valve 50,
from which, depending on the valve position, the wastewater may
flow via conduit 51 to a storage tank 52 and then via conduit 53 to
discharge to a waterway or a treatment facility (not shown), or
alternatively, the treated wastewater is returned via conduit 54,
collection tank 31, and conduits 34 and 35 for additional oxidation
cycles.
Electrolysis in such a cell reduces COD in typical wastewater by
oxidizing to CO.sub.2 the water-soluble organic and other
oxidizable contaminants of the wastewater. Typical purification
levels are in the range of 80-90% reduction in COD for one pass
through the electrolytic oxidation cell, and can exceed 95% COD
removal for recirculation systems, or in multi-cell wastewater
treatment systems of the invention.
The features and advantages of the invention are more fully shown
with reference to the following non-limiting examples.
EXAMPLE 1
Treatment of Wastewater from a Power Generation Station
Wastewater from a power generation station with a COD reading of
100 ppm was treated in an electrolytic oxidation apparatus of a
type as depicted in FIG. 2. The initial pH of the wastewater was
9.8 and its initial conductance was 1280 .mu.S/cm. The wastewater
was pumped through the cell at a rate of 1.2-2.6 liters/minute. The
voltage was held at 6.0 V and current was 12.5 A. After 5 minutes,
the outlet stream was tested for COD with a result of 20-40 ppm,
corresponding to a COD reduction of 60% to 80% in the effluent
discharge stream, relative to the influent stream COD level.
EXAMPLE 2
Treatment of Wastewater from Printed Circuit Board Manufacture
Wastewater from printed circuit board manufacture with a COD
reading of 200 ppm was treated in an electrolytic oxidation
apparatus of a type as depicted in FIG. 3, employing two cells in a
series arrangement. The initial pH of the wastewater was 9.8 and
its initial conductance was 1300 .mu.S/cm. The solution was pumped
through the cell at a rate of about 1 liter/minute. In the first
cell, the voltage was held at 6-8 V and current was 9-12 A. In the
second cell, the voltage was 10-12 V and the current was 9-12 A.
After passing through the two cells, the outlet stream was tested
for COD with a result of 20-50 ppm, corresponding to a COD
reduction level of 75% to 90% in the effluent discharge stream,
relative to the influent stream COD level.
EXAMPLE 3
Treatment of Wastewater from Landfill Seepage
Wastewater which had seeped from a garbage dump landfill site with
a COD reading of 250 ppm was treated in an electrolytic oxidation
apparatus of the type as depicted in FIG. 2. The initial pH of the
wastewater was 9.0 and its initial conductance was 1580 .mu.S/cm.
The solution was pumped through the cell at a rate of 1.2-4.0
liters/minute. The voltage was held at 9-9.5 V and current was
4.0-5.0 A. After 5 minutes, the outlet stream was tested for COD
with a result of 20-50 ppm, which corresponds to a COD reduction of
80% to 92%.
While the invention has been described herein with reference to
various illustrative features, aspects and embodiments, it will be
appreciated that the invention is susceptible of variations,
modifications and other embodiments, other than those specifically
shown and described. The invention is therefore to be broadly
interpreted and construed as including all such alternative
variations, modifications and other embodiments within its spirit
and scope as hereinafter claimed.
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